Benchmark Testing of ONERA and DLR Pressure Sensitive Paints for Unsteady/Instationary Flows

Aim: Some years ago, in the framework of the Motar Cooperation Agreement between ONERA and DLR, it had been agreed to carry out comparison testing of the two instationary PSP methods on a common test configuration with an unsteady flow produced in a suitable wind tunnel. The results from this comparison would form the basis for a benchmarking of the two different approaches, leading to an aid in decision-making as to which method is better suited for a particular application. / Test configuration: For many years there has been interest at ONERA on the (quasi) two-dimensional unsteady transonic flow in a channel with strong interactions between an oscillating shock and a separated boundary layer: various diagnostic methods were used in measurements on their S8Ch transonic wind tunnel at ONERA Meudon (Schlieren, pressure sensors, PIV, LDV), including also PSP with Ru/AA paint (uPSP). Flow separation of the boundary layer is produced over a convex contour profile (a bump) on the floor of the tunnel, with the subsequent flow acceleration leading to formation of a downstream shock wave. The sectional area of a second throat, situated downstream of the bump, is varied periodically (ca. 15 Hz or higher) by rotation of an elliptical rod (cam) placed downstream of the throat position, leading to forced upstream pressure fluctuations and hence an oscillation (movement in flow direction) in the shock position. The resulting pressure fluctuations on the bump can then be measured with the help of pressure sensors (Kulite) fixed in the floor, and also with PSP coated on the floor. / Procedure/Approach: The floor can also be coated with the DLR iPSP paint. As with ONERA, there exists sufficient optical access for placement of the DLR excitation LEDs and cameras adjacent to or above the test section. PSP measurements were then be carried out with a new set of measurements using both ONERA and DLR paints, consecutively. / Results: Pressure measurements with Kulite and iPSP/uPSP are compared. Phase correlation is good, but there remain some differences in amplitude for both paints. Static and instationary calibration were carried out on samples of both paints; the latter showed no phase loss up to 120 Hz, although the amplitude with iPSP dropped due to surface heating by the LED over time. Fourier analyses of Kulite and PSP showed good agreement. A table was compiled with a summary of characteristics for both paints: there is no “better” paint, since the choice is governed mainly by desired application.